RF screening by thin resistive layers

Abstract

We discuss the results of recent impedance measurements for an LHC dump kicker prototype, performed at CERN using the coaxial wire method. The kicker design includes a vacuum barrier consisting of a ceramic chamber internally coated with a thin metallic layer having good electric contact with the external beam pipe. For the bench test the coated ceramic tube was replaced by a kapton foil with a 0.2 µm copper layer having the same DC resistance of 0.7 Ω. The measurements show that this resistive coating provides a very effective RF screening down to frequencies below 1 MHz, where the skin depth is two orders of magnitude larger than the layer thickness and one could expect full penetration of the electromagnetic fields. We also present simulation results and analytic considerations in agreement with the measurements, showing that the return currents almost entirely flow through the copper layer down to frequencies where the reactive impedance of the kicker elements located behind it becomes comparable to the layer resistance. Finally we discuss the relevance of such coaxial wire measurements to the RF shielding by thin metallic layers in the presence of a higly relativistic proton beam.

Resistive Coatings on Layered Accelerator Structures with Rectangular Cross-Section

“…The measured reduction in coupling impedance was in agreement with the theoretical predictions, although the final choice of high resistance was dictated by voltage holding requirements which is not addressed by theory. The rf screening by thin resistive layers was also confirmed by Caspers et.al [7]in coaxial wire measurements for the LHC dump kicker prototype. In a recent paper, Gluckstern and Zotter [8] presented an explanation based on wave reflections for the feature that a conducting layer thin compared to the skin depth is able to shield electromagnetic fields.…”

“…Obviously, the value of the step inductance cannot be found from the uniform solution of a wave guide. If the step is comparable to the length of the discontinuity, the interference between the step induced fields lead to pill-box resonances, which show up at higher frequencies and require finite length treatment [7,8]. Although the plate model is not intended for a pill box analysis, it still can be used to estimate its impedance in the presence of a coating layer.…”

Transverse broadband impedance reduction techniques in a heavy ion accelerator

“…The transition pieces are often used to reduce the impedance of bellows, holes and so on. A high conductivity coating is often used to reduce the impedance of collimators, beam instrumentation and numerous other devices [14,15]. According to the transverse broadband impedance for the components of BRing in the Table I, ceramic rings in the vacuum chamber are the primary source.…”

“…It has been shown that a thin layer of high conductivity material plated on the surface of a poorly conductive material can effectively screen the beam from interacting with the poorly conductive material for a large frequency range [14,16]. In the BRing, the ceramic ring made of ZTA is a poorly conducting material and will generates a larger broadband impedance.…”

Beam coupling impedances for perforated beam pipes with general shape from impedance boundary conditions

“…The effect of so many holes/slots on the beam dynamics and stability, e.g. in terms of coupling impedances, is a fundamental issue and has been carefully investigated, both theoretically [2]- [6], and experimentally [7], [8].…”